1. Field of the Invention
The present invention relates generally to electronic navigation systems and more specifically to radio receivers for use in such systems.
2. Description of the Prior Art
Various electronic navigation systems are well known in the art. Loran systems, for instance, have found wide acceptance and have proven to be a valuable aid to navigation. More recently, a Global Positioning Systems (GPS) has been proposed wherein a constellation of satellites will be positioned in orbit around the earth. Each satellite transmits data which is modulated by pseudorandom noise code (PRN) peculiar to that satellite, thus resulting in a spread spectrum transmission which can be identified and processed by the user to determine his position. Such a system is described, for instance, in co-pending U.S. patent application Ser. No. 321,945 entitled "Global Positioning System Receiver", filed Nov. 16, 1981 and assigned to the applicant's assignee.
The receiver of the present invention retains the advantages of prior GPS receivers but provides the ability to make very accurate frequency measurements without requiring a precision local oscillator.